The allegations spread quickly through the Spanish media on Tuesday after El Mundo newspaper published the contents of a taped conversation purportedly of the group's leader discussing an offer from the king of two million euros (£1.74m) in cash to drop the case.

In the recording, a man identified as Miguel Bernad of the organisation Manos Limpias (Clean Hands) is heard describing an approach by an intermediary allegedly representing King Juan Carlos, requesting that the group withdraw its accusations of tax fraud against the princess in exchange for the payment.

“He made me an offer as follows: two million euros and, on top of that, all the expenses there may have been in connection with the trial […] all in cash," he alleged.

The voice reported to be Mr Bernad’s said that the alleged go-between he spoke to was Íñigo Cotoner, the Marquis of Mondéjar, whose father was a counsellor to King Juan Carlos in the 1970s and 1980s.

“He made the proposal on behalf of the emeritus king […] Juan Carlos”, the male voice confirmed to his surprised interlocutor, apparently Virginia López-Negrete, the lawyer who led the private prosecution by Manos Limpias against the princess at the trial which ran from January to June.

The public prosecutor asked for Mr Urdangarin to be jailed for 19.5 years. Princess Cristina stands accused only by Manos Limpias, which asked for a prison term of eight years.

The judge has yet to deliver her verdict.

The suggestion in the recorded conversation, dated at December 10, 2015, is that King Juan Carlos was allegedly willing to pay for his daughter to escape having to testify and being judged if Manos Limpias admitted it could not prove its case at the start of the trial.

The female voice said to be that of Ms López-Negrete is heard saying: “My answer is clear: absolutely not. Neither you nor I are in this for money. What a disgrace they are, what sons of bitches.”

The Royal Household declined to make any comment on the matter when approached by the Telegraph. El Mundo said the Marquis of Mondéjar had also refused to comment.

The 74-year-old Mr Bernad has been in jail since July while he is investigated over accusations of extortion for taking money from individuals and companies which Manos Limpias had accused of wrongdoing.

According to Mr Bernad, Manos Limpias is a union representing more than 5,000 civil servants, although the exact nature of the litigious organisation’s funding had been the subject of speculation for years.

Before his arrest, Mr Bernad told The Telegraph that Juan Carlos, who abdicated in favour of his son, King Felipe in 2014, had made behind-the-scenes manoeuvres to try to disrupt the judicial process.

“You have to weigh a parent’s love for their child who is having trouble against the damage that can be done to the royal institution," he said. "I think it is good for the credibility of Spain and the monarchy that the princess sits in the dock and has the chance to demonstrate that she is innocent”.